A Toscanini or a Karajan would either have quit or mounted a boardroom coup. But Bernard Haitink has soldiered gamely on as music director of the Royal Opera through 15 years of managerial mayhem, maintaining musical standards far higher than those of his administrative brethren, and braving a dizzying sequence of changes at the top while seeing the company into its proud new home before finally handing on his baton.

Where others might have flounced out, or publicly dissented, he has conducted new productions of which he clearly disapproved, such as Richard Jones's Ring (amid many more he clearly loved, including Johannes Schaaf's Figaro and Jonathan Miller's Così). Haitink has not so much risen above the Covent Garden chaos as remained curiously detached from it, quietly getting on with his job while most around him made a mess of theirs.

Whether this was strength or weakness, or a confused combination of the two, he has delivered all that has been required of him - and more - in the orchestral and vocal departments. How fitting, therefore, that Haitink bows out with a majestic Tristan und Isolde now midway through six farewell performances, punctuated this afternoon by a concert in his honour led by soloists from the orchestra he has coaxed and cajoled into the mean machine, well worthy of a world-class house.

Give or take two dinner intervals, Tristan amounts to four hours of foreplay building to a glorious 20-minute multiple orgasm. From the passion with which he holds this huge piece together, one suspects that Haitink does not share the widespread reservations about Herbert Wernicke's two-year-old production (revived by Halina Ploetz). Nor do I. Wernicke's use of primary colours to signify emotion is actually quite old-fashioned, very much in the postwar Bayreuth tradition of Wieland Wagner, the composer's grandson; his use of giant geometric boxes to keep the lovers apart reflects the central musical truth that they interact and commune less than yearning for each other from a tragically doomed distance.

The production's slow burn towards its necrophiliac Liebestod in fact plays happily to Haitink's musical strengths. Where Jones broke the Ring down into scrappy, irritating fragments, Wernicke maintains an intense, at times thrilling, tension, scrupulously mirroring both text and score as this epic tale wends towards its sumptuous climax.

If anyone steals the evening from Haitink, it is the Australian soprano Lisa Gasteen, whose Isolde must amount to one of the most memorable debuts even in Covent Garden's resonant history. A decade since she was named Cardiff Singer of the World, Gasteen has matured into a full-fledged Wagnerian par excellence; her effortlessly powerful voice caresses as much as it commands, as rich as it is colourful, maintaining its precision across an astonishing range - unmatched, alas, by her Tristan, the German tenor Wolfgang Muller-Lorenz. Where Gasteen soared majestically above Wagner's massive orchestral swells, her lover was sadly drowned out, his thin, inexpressive tenor simply not up to the task. It was also a serious mistake to remove his shirt, thus shedding any last pectoral pretence at heroic status.

Strong support came from Petra Lang as an eloquent, beguiling Brangane, Alan Titus as a doughty Kurwenal and Robert Lloyd as a duly commanding King Marke. The orchestraI playing is uniformly magnificent in all its many departments. If Isolde's final rapture is to mark Haitink's farewell to Covent Garden, his long and storm-tossed reign could not end on a stiller, stronger, more satisfying note.

The same can be said of the 'Discovering Handel' season mounted on the South Bank by the Orchestra of the Age of Enlightenment, which ended on Wednesday with a rare chance to hear his first oratorio, Il Trionfo del Tempo e del Disinganno. Written in Italy when he was only 21 (and attempting to master the finer points of Italian opera), Il Trionfo was also Handel's last oratorio, as he pillaged and tinkered with it all his long life, lifting motifs for his operas and reinvent ing this sparkling early work for later audiences. Two years before his death in London in 1759, blind and infirm in his mid-seventies, he revised it one last time to an English text as The Triumph of Time and Truth .

And no wonder this youthful piece always bore such a special place in Handel's heart. Amid signs of an aspirant straining for effect are sudden bursts of breathtaking energy and beauty fully worthy of the mature composer, at this stage mere portents of the glories ahead.

Given an unremarkable libretto from the Papally-favoured Cardinal Benedetto Pamphili, whose prim morality is mirrored in Beauty's surrender of her virtue (with a disappointing lack of struggle) to Time and Disillusion, the young Handel rose above his material in a patchwork quilt of set pieces offering both orchestra and soloists virtuoso opportunities.

Rinaldo Alessandrini's ensemble milked these riches for all they were worth, perhaps more, leaving a rapt QEH audience stunned by the limpid beauty of the still, slow closing aria after so much demonic energy and passion. As Bellezza (Beauty), the soprano Joanne Lunn gamely stood in at the last minute for the indisposed Sophie Daneman, with the consequent loss of four arias and some recitatives, and the tenor Nicholas Sears made a suitably stolid Tempo (Time). But the stars of the evening were the Italian contralto Sara Mingardo as Disinganno (Disillusion) and soprano Roberta Invernizzi as Piacere (Pleasure), both relishing their respective tasks of tugging Bellezza in different directions.

A truly trionfale note on which to end an imaginatively programmed season of Handel's lesser-known works. It has proved a worthy counterpoint to the composer's renaissance in the world's opera houses - also launched, surely, in England, with Nicholas Hytner's trailblazing Xerxes for ENO inspiring such successors as Glyndebourne's (and others') very different stagings of Rodelinda, Theodora et al. Long may he reign.